ADSORPTION OF CATIONS ON HYDROUS OXIDES OF IRON .1. INTERFACIAL BEHAVIOR OF AMORPHOUS FEOOH AND BETA-FEOOH (AKAGANEITE) IN DIFFERENT ELECTROLYTE-SOLUTIONS

The interfacial behavior of two hydrous oxides of iron, namely an amorphous variety (amor. FeOOH) and a synthetic akaganeite (β-FeOOH), in different electrolyte media has been studied by potentiometric titration (both rapid and batch procedures) and particle electrophoresis methods. The background electrolyte medium was varied from simple KNO3 and NaCl solutions to a complex electrolyte solution resembling seawater. In KNO3 solution the pHpzc and the pHiep are almost the same for both samples. In NaCl solution, the pHiep shifts by about a pH unit toward the acid side for amor. FeOOH due to specific sorption of anions; for β-FeOOH the pHiep shifts slightly toward the alkaline side due to the small specific sorption of cations. Consequently, the pHpzc of amor. FeOOH in 0.5 M NaCl, although it shifts slightly to the acid side in the presence of MgCl2, has no corresponding shift to the alkaline side observable in the presence of Na2SO4 solution. Calculations for surface species in different electrolyte media suggest that while surface ionization is stronger than complexation for amor. FeOOH, surface complexation is the stronger phenomenon contributing to the total surface charge on β-FeOOH. All these observations are in conformity with the ΔpKint a value of amor. FeOOH being smaller than that of β-FeOOH. © 1994 Academic Press, Inc.